As Republican Sen. Richard C. Shelby of Alabama prepares to defend his conservative credentials against what could be one of the toughest primary challenges of his Senate career next Tuesday, a conservative national group has thrown its support to him.

The Tea Party Patriots Citizens Fund – a political action committee that arrived on the scene in 2009 as the tea party movement was taking off – offered its last-minute stamp of approval on Thursday, opting against his chief rival, Marine veteran Jonathan McConnell, just five days before Alabama voters head to the polls. In a statement that praised Shelby's opposition to the 2008 Wall Street bailout and his efforts against illegal immigration, Jenny Beth Martin, the group’s chairman, said Shelby has “earned” another term.

Liz BeShears, a spokeswoman for McConnell, said, "We are unsurprised that a number of endorsements are going toward the incumbent, but it is the people who have the power in this race, not special interests."

The nod from the group – which joins others, like the National Rifle Association and National Right to Life, in backing the fifth-term senator – came after Shelby has already blazed an expensive path he hopes will take him to victory. He has spent $6 million in three months to boost his own conservative image and, more recently, to tear down his opponent's.

While Shelby, a former Democrat, has faced mild opposition before, what is different this time is that he will appear before an electorate that seems ready to support a political outsider such as Donald Trump at the top of the ticket. Shelby must get 50 percent of the vote to avoid a run-off.

His trepidation about this race has been apparent on Capitol Hill, where he has stalled the nominating process for presidential appointees who have appeared before the Senate's committee on banking, which he chairs. “My primary is Tuesday!” Shelby told the New York Times this week when asked about why their nominations were slowed. “We can talk about this later!”

The battle between Shelby and McConnell has intensified as the race moved closer to the primary. After weeks of promoting his own image, Shelby’s campaign released a scathing commercial attacking McConnell on illegal immigration earlier this month.

Just this week, he unveiled a new spot that questioned McConnell's ethics – pointing to allegations that McConnell tried to bribe one of the other candidates out of the Senate race and shed new light on an old story from McConnell’s college newspaper that accused him of “cheating in his college elections.”

McConnell's rivals have tried to raise the college elections issue earlier in the race. It has to do with a race for student government president at Auburn University in 2003, and accusations that his campaign attempted to get student ID numbers and PINs to influence the vote count. However, after an investigation, the election results stood.

The opponent who brought the challenge at Auburn was Michael Joffrion. Joffrion also is a Republican political adviser who has worked at the National Republican Senatorial Committee, which is now supporting Shelby for re-election. (Earlier this month, the group's executive director expressed confidence in Shelby's prospects on March 1.)

While McConnell – whose campaign has less than $180,000 in the bank, after raising $766,000 since October – lacks the financial strength of Shelby's $17 million campaign fund to fully rebut the attacks. BeShears said in one sense, the campaign actually views the negative attention as a potential positive.

"It appears that the wildly false and misleading attacks on Jonathan McConnell have backfired. We're less than a week out, and a 37-year incumbent going negative with practically endless resources seems to only have raised McConnell's name ID," she said. "I don't know how many times we've heard people be incredulous that an incumbent of Shelby's stature went that low."